1. Field of the Invention
The invention relates to the iron and steel industry, particularly to heating stoves of blast furnaces.
2. Prior Art
Each blast furnace is equipped with two or more heating stoves used for heating the blast before delivering it into a blast furnace. The conventional methods of operating such stoves are the alternative and the parallel (which here and below will mean staggered parallel, that is, parallel displaced in phase).
According to the first method, the alternative, at any given moment (not taking into consideration the short periods of connecting the ready to go on -- wind stove to the hot air manifold) the blast is heated in only one stove, while at the same time all of the other stoves are in their periods of heating. In this first method the stoves are connected to the blast furnace alternatively, one after another, in some definite pre-established order.
A time, temperature and pressure diagram of the group of stoves working alternatively is shown in the FIG. 1. The lines A.sub.1 B.sub.1 D.sub.1 C.sub.1, A.sub.2 B.sub.2 D.sub.2 C.sub.2, and so on, show the changing temperature of the blast as a function of time after the corresponding stoves No. 1, No. 2 and so on.
As shown in FIG. 1, at the end of the period of cooling of stove No. 1 (D.sub.1) stove No. 2 begins to be connected to the pipe of the cold blast (A.sub.2). The duration of this period of connecting (.DELTA..tau..sub.1) is usually 4 to 6 minutes. The section A.sub.1 B.sub.1, A.sub.2 B.sub.2, and so on, show how the losses of heat to the surrounding medium during the periods of pressurization influences the initial temperature of the hot air (T.sub.1).
The point B.sub.2 corresponds to the moment of connecting the stove No. 2 to the hot blast manifold. Simultaneously the stove No. 1 is disconnected from both manifolds of cold and hot blast (point C.sub.1) and after that, is connected to the atmosphere.
The line FF on FIG. 1 demonstrates the changing of the pressure of the blast air as a function of time. The diagram of FIG. 1 shows that during the periods of switching the stoves (sections E.sub.1 C.sub.1, E.sub.2 C.sub.2 and so on) the pressure of the blast drops. This drop of pressure is caused by the extraction of part of the air from the cold blast manifold to pressurize the stove being prepared to go on -- wind.
Usually the drop of pressure, .DELTA..rho., is 4 to 5 PSI. It is evident that during the periods of filling the stoves, the blast furnace receives less air than during other periods of time. These periodical drops in flow rate of the blast going to the furnace, which are accompanied by drops of pressure, adversely affect both the stability of operation and the efficiency of the blast furnace.
The optimal duration of the periods of switching is determined empirically by furnace engineers. Reducing this duration of the switching period would result in sharper drops of both the pressure of the blast air and the flow rate to the blast furnace, which is likely to cause serious disturbances to the blast furnace process. Increasing this duration would, on the other hand, result in increasing the thermal losses to the surrounding medium.
Under the staggered parallel operation of the stoves, as distinct from the above described method, at any given moment (except during the short term periods of pressurization) the blast air is heated in two stoves, which are operated in parallel, with displacement in phase approximately equal to half of the duration of the period of cooling of one stove.
A diagram illustrating this order of switching is shown in FIG. 2. Usually the period of time between two successive switchings of the stoves (.tau..sub.2) under the staggered parallel operation is less than under the alternative operation (.tau..sub.1), and the period of filling the stoves (.DELTA..tau..sub.1) is the same for both methods. Switching stoves under the staggered parallel operation has the same negative consequences which take place with the alternative method (see line DD, FIG. 2).
A number of methods are known for reducing or eliminating the drops of pressure during the periods of switching stoves. One particular method of fast pressurization of the stoves is by compressed air from an independent source, for example, an intermediate accumulator. This accumulator contains air compressed to a considerably higher pressure than the normal pressure of the blast air. The pressure in this accumulator is maintained by a special auxilliary compressor.
No one of the known methods solves the problem of using the compressed, still heated air, which is in some cases enriched by oxygen, that is still contained within the worked off and already cooled stove.
The energy of this air can be used differently depending on the method of operating of the stoves (alternative or staggered parallel). In the case of staggered parallel operation, the maximum effect will be from the method which combines the solution of the above mentioned problem with the use of an independent source of compressed air.